The invention relates to a filtering device comprising at least one passive filtering cell connected to a line of an electrical power system in which disturbance harmonic currents are liable to flow, and to an electrical apparatus comprising at least one such device.
Filtering devices are used to eliminate or attenuate harmonic currents or voltages in electrical power distribution systems. Harmonic currents are caused by non-linear loads connected to the power distribution systems. The most common disturbance loads are chopping power supplies, and converters or regulators comprising electronic power components.
An increase of the harmonic current or voltage ratios in the power distribution system is liable to disturb operation or to damage sensitive electrical loads. To limit the harmonic current or voltage ratio, it is known to use filtering devices connected on lines of the power system.
Filtering devices generally comprise passive filters and active filters. The passive filters operate as current sinks and absorb the harmonic components of frequencies appreciably equal to the resonance frequencies of said filters. The active filters enable a sinusoidal current to be reestablished in the power system by compensating the currents deformed by non-linear loads. Compensation is performed by injecting a correction current in parallel into the power system. The power system current then corresponds to the sum of the deformed load current and of the correction current.
Passive filters generally comprise filtering cells each comprising an inductor in series with a capacitor. Each filtering cell is tuned to a harmonic frequency of the disturbance currents. To be efficient, these filters have to be perfectly tuned to said harmonic frequency. But it is very difficult to obtain high-precision passive components for high powers. Moreover, the characteristics of the components are liable to change with aging or when the operating temperature varies. Another difficulty linked to the use of passive filters creates interactions between the cells and modifies the tuning frequencies of said cells. Likewise, connecting reactive power compensation capacitors on the lines creates disturbances in the filtering devices by modifying the tuning frequencies. Narrow-band passive filters can also lose their efficiency if the fundamental frequency of the power system varies, even in small proportions.
Active filters are less sensitive to electrical installation configuration variations as they are generally provided with servo-controlled control devices. However, active filters are limited in power because of the electronic components they comprise. Linear electronic power components, such as transistors, are in fact limited in voltage and current.
Hybrid filtering devices exist comprising passive filters and active filters. However, these devices are complex and costly. They are generally designed for very specific applications.